Mastering the rheology of a paint, both during the stage of its manufacture and during its transportation, storage, or implementation currently remains a priority. The diversity of the constraints within each of these steps is due to there being a variety of different rheological behaviors. However, the need of the person skilled in the art may be summarized as achieving the effect of thickening said paint, both for reasons of stability over time and for a possible application onto a vertical surface, the absence of splashing at the time of implementation, etc. This is why additives which contribute to this regulation of the rheological behavior have been designated as thickeners.
Historically, since the 1950s gums and cellulose-based products have been used, whose high molecular weight is one of their essential characteristics. However, these compounds exhibit a certain number of disadvantages, such as their instability over time (see the document U.S. Pat. No. 4,673,518), the need to use a large quantity of them (see the document EP 0,250,943 A 1), or their production costs, particularly in waste processing (see the document U.S. Pat. No. 4,384,096).
Next came so-called “associative” thickeners: these are water-soluble polymers that have insoluble hydrophobic groups. Such macromolecules have an associating nature: once added to water, the hydrophobic groups may assemble into micellar aggregates.
These aggregates are bound to one another by the hydrophilic parts of the polymers: a three-dimensional network is thereby formed, which causes the medium's viscosity to increase. Their operating mechanism and characteristics are currently well-known and described, for example, in the documents “Rheology modifiers for water-borne paints” (Surface Coatings Australia, 1985, pp. 6-10) and “Rheological modifiers for water-based paints: the most flexible tools for your formulations” (Eurocoat 97, UATCM, vol. 1, pp 423-442).
These associative thickeners include the category of HEURs (Hydrophobically modified Ethylene oxide URethane) and HASEs (Hydrophobically modified Alkali-soluble Emulsions). The former designate polymers resulting from the synthesis between a polyalkylene glycol compound, a polyisocyanate, and an alkyl and/or aryl associative monomer made up of a hydrophobic terminal group. The latter designate polymers of (meth)acrylic acid, an ester of these acids and an associative monomer made up of an oxyalkylated chain terminated by a hydrophobic group.
HEURs are the cause of many properties in aqueous paints, particularly as a function of the nature of their associative monomer. The following patent applications filed by the Company COATEX may be cited: EP 0,639,595 A1, which proposes hydrophobic groups having 4 to 36 atoms of carbon to increase the Brookfield™ viscosity, WO 02/102868 A1 which describes the use of polystyrylphenols with more than 40 atoms to increase the viscosity regardless of the shear gradient, and finally EP 1,425,325 A1 which discloses an associative monomer made up of di- and tristyrlphenol, make it possible to achieve an excellent pigment compatibility and a high viscosity at a low and average shear gradient.
However, the HEURs are chemicals which are difficult to dissolve in water, and must be placed in a solution in the presence of solvents or surface active agents, once their % of active ingredients exceeds about 25%. This problem is recounted in the document EP 0 682,094 A1: the proposed solution trivially relies on the use of surface active agents. Additionally, it is noted that HEUR thickeners, highly concentrated and containing surface active agents, had been sold by the company COATEX™ under the names COAPUR™ 5035 and COAPUR™ 6050 before the priority date of this application, and since 1993 for the first ones.
However, the implementation of solvents and surface active agents for formulating the HEUR thickener does not come without a certain number of problems. As a reminder, the solvents are subject to increasingly draconian legislation intended to restrict and even prohibit their use in paints. As for the thickening formulation's surface active agents, they are of such a nature as to destabilize the pains by interacting with the other surface active agents contained within them. Consequently, the formulation of HEURs in water is only possible provided that the polyurethane concentration is drastically limited (about 20% by weight), which makes these thickeners ineffective.
Thus, the person skilled in the art has turned towards HASE thickeners. For these thickeners, the choice of the associative monomer's hydrophobic group is the cause of various rheological properties. To that end, one may cite the following patent applications filed by COATEX™: EP 0,577,526 A 1, which describes of that chain with linear or branched alkyl and/or aryl groups having 26 to 30 carbon atoms, to develop high viscosities at a low shear gradient, and EP 1,778,797 A 1, which describes a branched terminal chain comprising 10 to 24 carbon atoms, to improve pigment compatibility and increase viscosity overall.
These HASE emulsions come in the form of an acid: it is well-known that they increase viscosity after neutralization, at a pH roughly greater than 6, whenever they are placed in the medium that they are intended to thicken. In concrete terms, they are added into this medium and a base is added which neutralizes the carboxylic sites of these emulsions: this triggers the solubilization of the polymer, and therefore the phenomenon of thickening, strengthened by the presence of hydrophobic groups of the associative monomer.
It is possible to neutralize these HASE emulsions first, meaning as they are (or before being added into the medium intended to be thickened) to a pH of around 6: but if this is done, the viscosity of these emulsions increases so much that they are no longer pumpable or workable. This is among the basic knowledge of the person skilled in the art, and is explicitly described in the document EP 0 013,836 A1 (page 4 lines 11-24 and page 8 lines 5-25).
The only exception to this rule concerns emulsions containing HASE polymers with alkylphenols, which can be found in pre-neutralized form, workable and with a solids content of about 25%. Nonetheless, alkylphenols are currently widely suspected of being carcinogenic and hazardous to reproduction; though still tolerated in the paint industry, they are still being watched by legislative institutions in the field, particularly European ones. These products therefore do not constitute a satisfactory response for the person skilled in the art.